Lignin-reinforced rubber



quantities in either sodium Patented Feb. 16, 1954 2,669,592 LlGNlN-REINFORCEJ) RUBBER onald W sl M G and Thomas ltaymond tario, cane H ward Smith ree Qu e j iies or, Cornw ll. Q terio.

a, .aslsign rs, by m e assi nments P p Mi ls himmed Mont No Drawing. Application August 24,1951, Serial No. 243,594

t) Claims. (01. {Mil-{756) This invention relates to improvements in the compounding of rubber, and to the production of lignin-reinforced vulcanizates having a satisfactory curing rate together with exceptionally high tensile strength, resilience and other valuable and distinctive properties.

The present invention is based on the discovery/that the use of oxides of {certain metals .in the compounding nice-precipitated ligninreinforced natural rubbers both shortens thecuring time and greatly enhances the "tensile strength, resilience and other desirable properties of the vulcanized products. It has been'iound that these oxides inhibit the delaying action of lignin on the vulcanization reaction. Theoxides found suitable for this purpose are oxides or m tals. l The sulphides of which will precipitate an acid solution, are insoluble except in small b lblhifi ammonium polysulphide and are stable the ,p jsence .of HCl and include copper oxide, 7 litharg'e, red lead, brown lead and bismdthtiirixide. The selected oxide or oxides may be addedflto the lignin before the latter is incorporated into the rubber 9r, alternately, the'joxide Ln'iay be Withheld from the co-precipitationQstep and added at he rubber mill. The la ter procedure :se m to give the best results and ispreferred.

In addition to producing l'i'gnin-reinforced atvural rubbers having a; satisfactory curing rate, r

the present invention greatly improves the. tensile strength, resilience and other properties ofriatural ligninn'einforced rubbers to an extent never before obtained. By the use of this invention it has been found possible to produce lignin-reinforced as high as and over 500021105. \per square inch and a Bashore resilience as :high as (65 Incomparison with this, thetensile strength of a standardcarbon-black formulation when prepared under th most iavcurebl condit ons. i l-arqhn 00 bspe square inc. with azmuo l wer Bash r r silien lli ninosucoe sfu ly used L h p ac ic of th inventionine udesaodailis and kraft gnin de t y e ..f r om s t an "har Woods.

Prior to our invention, :the-use oficopperi oxide Was considered "to .cause .poorlageing and :to i-be --d .let erious.torubbercompoflndsand was-avoided.

firhe use of litharge .in thev compounding ofjaru'bber has been practically 1 discardediinzfavour .of

'zincloxide. In .thecase. of .lignin-reinforced :natural rubber, the use of zinc oxide-inthe .:formulations previousl mployed ave hoof. .cul eiretherefore, quite contrary to "the l "ulcanized ubber having a tensile strength asj er en a g oxide increased as the amount of lignin is as lifihar may be 5 2 indi ations f p i r ar t a hing with ec 'tdthe'use of m'etalsin rubber compounding to discover that the addition of metals such as copper oxide, litharge, red lead, brown lead and bismuth t rioxide not only shortened the time of curing of'lignin-reinforced rubber, but' also substantially improved both the tensile strength andres'iliency' of the vulcanized product. Ligninerubber masterbatches used in producing ligninereinforced' 'vulcanizates' in accordance vw u i ve ti n ma .h p ej ared by add n 'analkaline solution of lignin to rubber latex and simultaneously coagulating; during agitation, the lignin andrubber with a precipitating solution containing sodium chloride and sulphuric acid or othe su t b e eo gu eh .Themixing .of ingredients is carried out in the usual manner w ll known in the art and may convenientlybe accomplished on a rubber mill. The ligninl-rubber masterbatch is homogenized inthe'usual way by placing the masterbatch on [the mill and continuing the milling until the rnasterbatchis homogenized, i. e., until the rubperforms a smooth sheet on the front roll and ,a bank in the nip of the rolls. At this stage the other compounding ingredients such as the metal ic oxide ulphu an ac a r ar "addedl' vulcanization may be carried out in a mold under pressure in the usual manner and at any convenient temperature at which vulcanization will take place; The temperature employed for the formulations given in the examples hereinvafterreferred to may be from 220 F. to 350 F. The figures'herein given for tensile strength, resilience, etc, of vulcanizates produced with such formulations were obtained by following t standard ro edu e om e ed i A- T- M- D-l '-f ,i

.We have iound it advisable to calculate the metallicoxide incur improved rubber compounds cc of the i n nhe a oun o 'increa's'ed. 'flfhe amount of lignin present ina formulation may be as low as about 5 parts by weight e l snin 150, 0 p rts by we ht .of rub- .bi ir. VI-l ht t loadings p a ed i t mi were of .Ithe' order of 250 parts by weight of lignm tolQO partsby weight of rubber. Higher lignin loadings are feasible where necessary or desirable The optimum loadjg of a metallic oxide such to 7 parts by Weight of th prid in a comp un c n i .1Qa

by weight of nihloljlqo ne sb wei h o mam rubber or about 17.5% based on total lignin in the compound. It is difficult to definitely fix a possible minimum amount of oxide present in a mix since even 1 part by weight of litharge to 40 parts by weight of lignin has been found to be beneficial. So far as can be determined, the lower limit of metallic oxide is probably around 0.5 parts (about 1%) by weight or lower of lignin.

The practical maximum amount of an oxide, for economical reasons, seems to be about parts or 37.5% by weight based on the total lignin per 100 parts by weight of rubber. However, beneficial effects may be obtained at higher oxide concentrations.

It is important to note at this point that the improvement in rubber afforded by our invention is due to the presence of a new composite compounding ingredient consisting of lignin and an oxide selected from the oxides of metals having the distinguishing characteristics hereinbefore specified, the oxide being present in the proportion of about 1 part by weight to 4 to 12 parts by weight of lignin. Other ingredients ordinarily employed in the compounding and vulcanization of rubber are also used in our improved formulations, as shown by various examples hereinafter referred to.

Zinc oxide has been used in earlier vulcanization processes as an activator and may be used in our process for the same purpose. Litharge, red lead and brown lead act in our formulations both as an inhibitor to overcome delaying effects "of lignin on the rubber cure and as an activator whereas copper and bismuth trioxide are not activators. Because of the superior activating qualities of zinc oxide, it is preferably used in the formulations even when lead oxides are prescut.

The optimum quantity of zinc dimethyl dithiocarbamate (Zimate) necessary in the presence of lignin is one part by weight to 100 parts by weight of a compound containing 40 parts of lignin, or 2.5% based on the lignin in the compound. As little as or less than 0.1 part per 100 parts by weight of rubber or 0.25% based on the lignin produces a beneficial effect in vulcanization of rubber.

The sulphur content in the formulation is also dependent on the total quantity of lignin... The content of sulphur in soft rubber containing lignin ranges from about 0.75% to about 14 92, based on rubber. To minimize the ageing it is preferred to keep the sulphur content at the lowest value consistent with satisfactory vulcanization. About 2 parts by weight of sulphur per 100 parts by weight of rubber is preferred when the lignin is not above 40 parts by weight per 100 parts by weight of rubber. It is beneficial to increase the sulphur content when the lignin is higher than 40 parts by weight per 100 parts by weight of rubber. Beyond this loading the sulphur should be increased by approximately 3% of any lignin above 40 parts by weight per 100 parts by weight of rubber.

The percentages given above are not necessarily constant. Such percentages are given merely to illustrate that the quantities of variable ingredients increase with the increase in quantity of lignin in the compound.

The invention is further illustrated by the following tables and examples.

Table 1 shows the practical percentages of metallic oxide, zinc dimethyl dithiocarbamate, and

sulphur, based on lignin'loadings'" TABLE 1 Optimum Minimum Maximum Ingredents Percentage Percentage Percentage Metallic oxide 14-20 0. 2 40. Zinc dimethyl dithio' 2.5 0.25 7.5.

carbamate. Sulphur 1 (in addition 3.0 0.10 6 (soft rubber).

to amount required for the rubber itself).

Based on any lignin over 40 parts by weight per 100 parts by weight of rubber.

The curing characteristics of natural rubber containing lignin and metallic oxide as described were obtained with the formulation shown in the following examples:

Example 1 Parts by weight Lignin masterbatch 150 Stearic acid 2 Phenyl beta naphthylamine (Neozone D),

antioxidant l Litharge 20 Zinc dimethyl dithiocarbamate (Zimate) organic accelerator 1.5 Sulphur 4 1 Is 50 parts by weight of lignin (to-precipitated with 100 parts by weight of rubber.

This formulation gave a cure in 15 minutes at 292 F. The tensile strength was 4160 pounds per square inch and the resilience was 58%.

A relatively large quantity of litharge was used in this example. The litharge in the next example was reduced and zinc oxide was added to the formulation as an activator of rubber vulcanization.

Example 2 Parts by weight Lignin masterbatch 120 Smoked sheet 20 Stearic acid 2 Phenyl beta naphthylamine (Neozone D), antioxidant Zinc oxide 3 Litharge 7 Zinc dimethyl dithiocarbamate (Zimate), or-

ganic accelerator N cyclohexyl 2 benzothiazole sulfenamide (Santocure) accelerator 0.5 Sulphur 2 40 parts by wei ht of lignin co-precipitated with 80 parts by weight of ru her.

2 Added to make rubber total equal to 100.

The time of cure in this experiment was 40 minutes at 272 F. The tensile strength obtained was 5040 pounds per square inch and the Bashore resilience was 49%.

In the following example is shown a formulation for various lignin loadings.

Example 3 general Parts by weight Lignin masterbatch Stearic acid 2 -Phenyl-beta-naphthylamine (Neozone D),

present in the rubber compound.

antioxidant l Zinc oxide 3 N-cyclohexyl 2 benzothiazole sulfenamide (Santocure) accelerator 0.5 Sulphur 2 Additional sulphur =3 Litharge 17 .0

Zinc dimethyl dithiocarbamate (Zimate),

accelerator 3 1 Natural rubberlignin-variable. This figure is a percentage based on any lignin above 40 parts by weight per 100 parts by wei ht ofrubber.

' These figures are percentages base on total lignin morass "this example.

Example 4 Parts by weight 1 Contains 36.1

parts'by weight of lignin.

The table below shows theeffectiveness of particular metallicoxides of equal quantity.

Parts by weight Lignin masterbatch ..V 1083 Smoked sheet U Mi n--- 127.8 Stearic acid. V 2.0 Zinc oxide v3 A, B, C, D or E 7-.0 Methyl Zimate accelerator 1.0 Santocure, accelerator l a, 0.5 Sulphur V ,V r p r r 2.0

As'in'Exampie 4. 2 As in Example 4.

= A1itharge; B-red lead oxide; Cbrown lead oxide;'

Dcopper oxide; Ebismuth trioxide.

Cure at *Bashore 282 F., 25 33 resilien r.-e

minutes (percent) A 10 4, 200 44 B 10 3, 075 45 C 10 Slight cure D 10 65 46 E s 3, 910 44 A 4, 800 48 B 20. 4, 100' 52 O 20 3, 465. 55 D 20 4, 48 0 48 E 20' 3, 720 46 A 4, 460 51 B 30 4, 150 53 O 3.0 4, 210 57 D. 30' 4, 045- 48 E 30 3, 555 46 A 40. 4, 495 50 B. 40 3, 855 52 C 4O 4, 100 60 D 40 4, 000 47 z: 40;, 545 46 Example 6 The table below shows the elfectiveness of various quantities of litharge at various times of cure.

,fcaustic. should be Continued oxidation is a decrease oo sb Lignin masterbatchi-,;,;, 108.3 Smoked sireet 27.8 Stearic acid I 2.0 Zinc oxide 3 A, l3, 0, D, E, F Variable Methyl Zimate l 1,0 ..:Santocur,e .0..5

Sulphur Cure Bashore (282 F), gresilience minutes (percent) A 10 2, 010 $15 -=B 10 2, 775 .47 C I0 '3, 280 49 D 10 7 3, 505 -51 E .10 3.9. F '10 I 4,285 L 57 A #291 ,3 9 x B 20, 3,.8 0 8 (3.- 2O ,175 53 o 220 4.390 a .5 E 2o peso 456 F 20 865 5 6Q A V 30 3, 170 46 B 30 3, 695 49 C v .30 10. 5 D 30 4, 385 54 E 30 4127.0 5 7 F. .30 4, 600 160 A. 40 2, 975 '1 B .40. 3, 405 48 C 110 3. 6.75 '50 D :10 3,865 52 E. 40. 13,0. 353 F 5:0. 4, 545 260 Patent 2, 6l-0,934-, Rudolf A. V. Ralf and George H. Tomlinson, Jr.,-h ave discovered that, in the compounding of lignin-rei-niorced rubbers, both natural and synthetic, the use of oxidized lignin -gives improved tensile strength as compared with the use of unoxidized lignin of the same source.

The oxidation of the oxidized ligni-n used in accordance with this invention may be carried out by bubbling air through an aqueous solution of lignin acid salt or lignin sodium salt or in other ways, such as by purposely oxidizing the {black liquor prior to or during its precipitation treatment or by oxidizing the 'lignin acid salt in dry powdered form in a current of hot air or oxygen. This may he carried out with or without catalysts or through the us .of specific ox dizing agents. The oxidative treatment, preferably done in a lignin solution maintained at a pH of around 10 by the occasional addition of until the meltin point .of the modified lignimlafterprecipitation and drying, is at least 240 C... Concomitant with in the methoxyl content and an increase in the viscosityofnon-agueous solution and for any given initial lignin, this property may ,he used lbesides the melting point as anindex of adequate oxidation.

The following example ,corunares the. tensil strength and resilience of lignin-rein'forced vulcanizates produced from compositions containing oxidized and unoxidized soda lignin from the same source; in each case the lignin being 36.1 parts per parts by weight of rubber and Zinc oxide otherwise compounded in accordance with Ex- The following example illustrates the effect of litharge in the vulcanization of masterbatches prepared by the co-precipitation of either unoxidized or oxidized kraft lignin with natural I rubber latex.

Example 8 Stenric acid Sulphur upw 1 Contains 36.1 parts by weight of either unoxidizcd or oxidized krait lignin.

1 Added to make rubber equal 100.

UNOXIDIZED KRAFT LIGNIN Without litharge both oxidized kraft lignin showed relatively low tensile strength.

With litharge in the formulation, the maximum tensile strength of the unoxidized lignin formulation was increased from 2545 p. s. i. to 4730 p. s. i. The maximum tensile strength of the oxidized lignin was increased from 2150 p. s. i. to 5255 p. s. i. The effect of litharge was, therefore, to increase the tensile strength and resilience with both types of lignin considerably.

These results show that litharge plus zinc oxide in a kraft lignin masterbatch formulation the unoxidized and the is much superior to zinc oxide alone, and that '8 the litharge formulation improves considerably both unoxidized and oxidized lignin.

In the formulations described herein the reinforcing agent or filler present was lignin. Other reinforcing agents that may be used in the presence of lignin are, such as, carbon black, clay, whiting, etc.

Having thus described the nature and various embodimentsv of our invention, it will be understood that the present disclosure is illustrative rather than limiting and that various modifications may be resorted to within the scope and spirit of the invention as defined by the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A re-inforced vulcanizate prepared by vulcanizing a composition comprising coprecipitated natural rubber and lignin in the presence of an organic accelerator, suflicient activator selected from the group consisting of lead oxide and zinc oxide to activate said organic accelerator, sulphur and a modifier efiective to inhibit the curedelaying action of the lignin, and. to substantially enhance the tensile strength and resilience of the said vulcanizate, said modifier being selected from the group consisting of the oxides of copper, lead and bismuth.

2. The vulcanizate claimed in claim 1 wherein the modifier is a lead oxide.

3. The vulcanizate claimed in claim 1 wherein the modifier is litharge.

4. The vulcanizate claimed in claim 1 wherein the modifier is red lead.

5. The vulcanizate claimed in claim 1 wherein the modifier is brown lead.

6. The vulcanizate claimed in the modifier is bismuth trioxide.

7. The vulcanizate claimed in claim 1 wherein the modifier is a copper oxide.

8. A method of vulcanizing a coprecipitated lignin natural rubber composition with comprises; compounding said composition with compounding ingredients including sulphur, an organic accelerator, an activator therefor selected from the group consisting of lead oxide and zinc oxide and a modifier selected from the group consisting of the oxides of copper, lead and bismuth; and then subjecting the resulting mixture to vulcanization.

9. A method as claimed in claim 8 wherein said compounding ingredients include zinc oxide as an activator for said organic accelerator.

DONALD WESLEY MAcGREGOR.

claim 1 wherein THOMAS RAYMOND GRIFFITH.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,217,157 Coughlin Feb. 27, 1917 1,844,306 Williams Feb. 9, 1932 2,355,180 Remy Aug. 8, 1944 2,572,884 Pollak Oct. 30, 1951 2,575,061 McMahon Nov. 13, 1951 2,608,537 Pollak Aug. 26, 1952 OTHER REFERENCES Rubber Age (New York) of November, 1948, pp. 197-200. 

1. A RE-INFORCED VULCANIZATE PREPARED BY VULCANIZING A COMPOSITION COMPRISING COPRECIPITATED NATURAL RUBBER AND LIGNIN IN THE PRESENCE OF AN ORGANIC ACCELERATOR, SUFFICIENT ACTIVATOR SELECTED FROM THE GROUP CONSISTING OF LEAD OXIDE AND ZINC OXIDE TO ACTIVATE SAID ORGANIC ACCELERATOR, SULPHUR AND A MODIFIER EFFECTIVE TO INHIBIT THE CUREDELAYING ACTION OF THE LIGNIN, AND TO SUBSTANTIALLY ENHANCE THE TENSILE STRENGTH AND RESILIENCE OF THE SAID VULCANIZATE, SAID MODIFIER BEING SELECTED FROM THE GROUP CONSISTING OF THE OXIDES OF COPPER, LEAD AND BISMUTH. 